update size calculations for assignment statements

this was forgotten when the value lists were extended by a size hint.

Change-Id: I6f9b55ed671224a9b8735c8d937f94aac4a73a42
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>
(cherry picked from qtcreator/f24c9865306624c2fc150d4bd262a5c4d5a3689a)
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>